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The backside of Diplodocus, photographed at the Utah Field House of Natural History. Photo by the author.

In 1991, the American Museum of Natural History unveiled one of the most fantastic fossil displays ever created. Placed at the center of the renovated Theodore Roosevelt Rotunda, an adult Barosaurus rears back to protect its offspring from an oncoming Allosaurus. The defending sauropod’s head is 50 feet up in the air, although whether or not such an immense, long-necked dinosaur could have pulled off such a feat has been a continuing point of contention. Even in a typical posture, Barosaurus must have had a powerful heart to pump blood along its 25-foot neck, and who knows how hard the dinosaur’s heart would have to work to continue bloodflow to the animal’s head if it reared up? Some paleontologists consider such a feat physically impossible, but as paleontologist William Gallagher pointed out while teaching my Paleontology 101 class at Rutgers University, male Barosaurus had a good reason to rear up. How else would the huge dinosaurs have positioned themselves to mate?

Exactly how dinosaurs got it on has inspired no small amount of speculation. The largest dinosaurs of all, the sauropods, have been especially perplexing. We often say that these dinosaurs “shook the earth” with their footsteps, but did they also make the bed rock with their lovemaking? (I apologize for that joke, and will keep the geology puns to a minimum. Promise.) Paleontologist Beverly Halstead famously wondered about dinosaur sex in public lectures and articles, and he suggested that standard “dinosaur style” was for a male to come alongside the female and throw its leg over the female’s back as she lifted her rump into the air to move her tail out of the way. In the case of sauropods such as Diplodocus, Halstead even imagined that the amorous dinosaurs might intertwine their tails. While other paleontologists have considered the tail-twisting aspect unlikely—sauropod tails were balancing organs and were too stiff to intimately coil around each other—the basic dinosaur position Halstead promoted has remained a prominent possibility for the dinosaur kama sutra.

But not everyone agreed that giants such as Apatosaurus and Brachiosaurus were capable of such nupital acrobatics. These animals were so immense—Apatosaurus, not even the largest sauropod, is estimated to have weighed more than 23 tons—that some researchers thought the kind of positions Halstead was promoting would give the dinosaurs fractured legs and broken spines. At a symposium of vertebrate morphologists held at the University of Chicago in 1994, biologist Stuart Landry, Jr. gave a short presentation entitled “Love’s Labors Lost: Mating in Large Dinosaurs.” He did not see how sauropods could have mated on land. A large, rearing sauropod, he told his audience, “would have to support 10 to 20 tons in a precarious position two or three meters off the ground.” A male Apatosaurus would be liable to tip over and possible take the female with him. Instead, Landry suggested that the largest dinosaurs looked for muholes or bodies of water to buoy themselves up. When a conference attendee asked if he was proposing that all dinosaurs mated in water, Landry responded, “I would say the very large ones must have.” Of course, this hypothesis required a large number of Jurassic and Cretaceous hot tubs of just the right depth for sauropods to reproduce, and scientific models of sauropods have suggested that these dinosaurs were actually quite buoyant and unstable in water. Sauropods were diverse, disparate and widespread animals that roamed in terrestrial habitats all over the world—there’s no reason to presume that the largest dinosaurs had to seek out the nearest deep lake when they got the itch.

Biomechanics expert R. McNeill Alexander also considered the weight problem in his 1989 book Dynamics of Dinosaurs & Other Extinct Giants, but came to a different conclusion. Even though a male sauropod would have rested a great deal of weight on the back of the female during mating, Alexander pointed out that the stresses and strains would not have been any more severe than those caused while the female dinosaur was walking. (After all, walking requires shifts in weight as the dinosaur balances and goes through each step cycle, and so a dinosaur’s skeleton had to be strong enough to cope with these shifts.) “If dinosaurs were strong enough to walk they were strong enough to copulate,” Alexander wrote. “They were presumably strong enough to do both.”

Without living specimens to observe, we will never know all the intimate details of sauropod sex. Still, there are only a limited number of positions that could have worked for the dinosaurs. For reasons that I’ll write about later this week, the consensus among paleontologists is that male dinosaurs probably had relatively small penises. (Shhh! Don’t tell Tyrannosaurus. He’s already upset about all those “useless forelimbs” jokes.)  An amorous male would have to position his cloaca—the orifice used for both expelling waste and mating in crocodylians, birds and probably dinosaurs—right up to the cloaca of a female, and the female’s tail would have undoubtedly presented an obstacle. Rather than simply leaning straight against the top of a female like an elephant or rhinoceros does, a male sauropod would probably have to rear up at a relatively oblique angle, and the female would have to assist by moving her tail (which is also a way in which female dinosaurs could have exerted mate choice and confounded any hot-under-the-collar males they would rather not mate with). Perhaps some museum will look into the problem and try to mount a pair of coupling sauropod skeletons—much like the mating Tyrannosaurus at Spain’s Jurassic Museum of Asturias—but even then we are limited by what we can imagine. Whether we want to imagine a pair of Brachiosaurus in flagrante delicto is another matter altogether.

This post is the first in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.

References:

Alexander, R. M. 1989. Dynamics of Dinosaurs & Other Extinct Giants. New York: Columbia University Press. pp. 57-58

Anderson, J. The Perplexing Puzzle of Maladroit Mating. Chicago Tribune. August 30, 1994.

The head and neck of "Gordo" the Barosaurus on display at the Royal Ontario Museum. From Wikipedia.

Stretching 90 feet long in life, Barosaurus was one of the largest of all dinosaurs. Despite its size, however, this sauropod was able to hide in the collections of the Royal Ontario Museum for over four decades.

Barosaurus were rare dinosaurs. One of the few skeletons ever found was uncovered by paleontologist Earl Douglass during his excavations of Utah’s Dinosaur National Monument in the early 20th century. As with many specimens from this site, the bones were sent to Pittsburgh’s Carnegie Museum of Natural History, but in 1962 they were traded to the ROM in Toronto, Canada.

The skeleton was thought at the time to be a Dipolodocus—which it does resemble, albeit with a proportionally longer neck and shorter tail. The ROM intended to include it in a revamped dinosaur exhibit set to debut in 1970, but the skeleton was left in storage due to a lack of floorspace. The sauropod expert Jack McIntosh later recognized the bones as belonging to Barosaurus, but after this point the skeleton simply sat in museum storage, effectively forgotten.

The bones were finally dusted off in 2007. With the ROM planning to open a new dinosaur hall, the museum assigned paleontologist David Evans, their new Associate Curator of Vertebrate Palaeontology, to find a sauropod skeleton for the exhibit. Evans investigated numerous options, from using a cast to finding a new specimen, and while searching for dinosaurs in Wyoming he came across McIntosh’s reference to a Barosaurus at the ROM. Evans immediately flew back to Toronto, and after a bit of searching he discovered the dinosaur’s lost skeleton. While not entirely complete, the dinosaur was represented by both femurs, both upper-arm bones, four neck vertebrae, the complete set of back vertebrae, fourteen tail vertebrae and other assorted parts.

Finding the skeleton was only the first challenge. The second was putting it all together in time for the opening of the new dinosaur hall. Evans had only eight weeks to do so, and this included creating casts of all the missing parts. The team of paleontologists and reconstruction experts was able to pull it off, though, and today the Barosaurus—nicknamed “Gordo”—looms over the ROM’s dinosaur hall. It was recently featured on the miniseries Museum Secrets, and the show’s web site includes several video clips about the behind-the-scenes work put into Gordo’s assembly.

Back in 1991, paleontologists at the American Museum of Natural History in New York City created one of the most ambitious and controversial dinosaur exhibits ever seen. An homage to the (at the time) new vision of dinosaurs as active, dynamic animals, the skeletal scene depicted an Allosaurus menacing a juvenile Barosaurus, with the young dinosaur’s long-necked, whip-tailed mother rearing up on her hind legs in defense. This put the adult sauropod’s head about 50 feet in the air—an altitude that the dinosaur’s heart may not have been able to handle, given the distance blood would have to travel from its chest to its head. But regardless of the ongoing debate over sauropod biology the mount represents, it is an impressive sight.

Now, after nearly two decades, a rift is developing between the famous battling dinosaurs. After noticing a lot of visitor-induced wear at the margins of the exhibit, the museum’s staff decided to open up a pathway through the exhibit so that museum patrons can walk between the dinosaurs. For the first time, they will be able to get right in the middle of a prehistoric confrontation never witnessed by any human.